A number of techniques are conventionally known that are capable of fractionating particles in limited circumstances. For example, one such technique involves the use of a microfabricated sieve consisting of a two-dimensional lattice of obstacles or barriers for DNA separation. The asymmetric disposition of obstacles or barriers rectifies the Brownian motion of DNA molecules that pass through the sieve, causing the particles to follow paths that depend on the respective diffusion coefficients of the DNA present. Although moderately effective, this technique includes a number of limitations. For example, because the lattice is microfabricated, the overall structure is capable of neither tuning nor adjusting the types and sizes of particles that are fractionated. Furthermore, such lattices tend to suffer from clogging, requiring flushing of the system and restarting.
Further, many conventional techniques for fractionating particles achieve physical separation of the various fractions along the direction of an applied force. For this reason, they operate on discrete batches of samples, and do not operate continuously.